INVESTIGATION OF THE INFLUENCE OF SOLAR CELL PROCESSING ON STRUCTURAL DEFECTS IN HPMC-SI WAFERS BY PHOTOLUMINESCENCE IMAGE ANALYSIS

摘要

In this paper we investigate the effect of selected processing steps used in Si solar cell fabrication on the recombination activity of grain boundaries (GBs) and other structural defects in high performance multicrystalline silicon (HPMC-Si) wafers. Such defects can in many cases be improved or completely removed using suitable processes, such as impurity gettering and hydrogen passivation, which is accomplished during existing solar cell production. Here we aim to both improve existing production processes with respect to bulk lifetime, as well as introduce new, dedicated processes designed for defect engineering and material quality improvement of HPMC-Si wafers, thus working towards closing the remaining efficiency gap between multi- and monocrystalline Si wafers. Groups of neighbouring wafers from different HPMC Si ingots, have been passed through various processing steps, and subsequently etched down and re-passivated to assess the bulk lifetime of the final solar cells. Photoluminescence (PL) lifetime imaging are then used to generate lifetime maps before and after a selected contact firing process to investigate the effect of GB hydrogenation during this process. The recombination strength of GBs was quantified by numerical simulations of charge carrier diffusion based on line scans of the carrier lifetime. A simplified performance metric was then used to quantify and correlate the recombination of a large number of GBs to the crystallographic properties of the GBs, measured by a Laue scanner on neighbouring wafers.